SM series of sub-Electric insulators classified Class A (copper nut is 1.5 times the thickness C grade copper nut, torque and tensile strength of the C-class doubled For retardant please specify.) Class B (for iron-plated nuts nickel, torque and tensile strength is a double A-level recommended) Grade C (for thin copper market is the most common nuts, the price advantage, the proposed less stringent requirements for quality clients only.)